The present invention relates to a wedge plate type light guiding plate for a front light and a method for manufacturing the light guiding plate.
To reduce the power consumption, some conventional display units have a lighting device. When there is sufficient outside light such as sunlight or room lighting, this type of display unit uses outside light. When the outside light is insufficient, the display unit uses the lighting device. Such display units include a reflective display unit with a front light.
FIG. 6 shows a reflective display unit 50 with a front light. The display unit 50 includes a light source 51, a light guiding plate 52, and a reflective liquid crystal panel 60. The light guiding plate 52 has an incident plane 54, a first exit plane 53, and a second exit plane 55. The liquid crystal panel 60 is located below the first exit plane 53. A considerable number of grooves 57 having saw-tooth cross-section are formed on the second exit plane 55. Each groove 57 is defined by a light admission plane 57a and an inclined plane 57b. The light admission plane 57a is inclined by an angle xcex3 relative to the first exit plane 53. The inclined plane 57b is inclined by an angle xcex1 relative to the first exit plane 53. The light admission plane 57a and the inclined plane 57b form an apex angle xcex2.
To improve the visibility of the reflective liquid crystal panel 60, it is desirable that the brightness of the panel 60 be enhanced and the brightness distribution of the panel 60 be uniform.
Brightness uniformity is an index representing the evenness of a brightness distribution. To compute the brightness uniformity, the values of brightness at specific points on the light guiding plate 52 are measured. The brightness uniformity is computed based on the maximum value and the minimum value of the measured brightness. Specifically, the brightness uniformity is computed in the following manner. First, the surface of the light guiding plate 52 is divided into sixteen sections by three parallel lines and another three lines perpendicular to the first three lines. The brightness at each of the nine intersection points L1 to L9 is measured. Then, the minimum brightness is divided by the maximum brightness. The resultant represents the brightness uniformity. The closer to one the resultant is, the smaller the difference between the minimum brightness and the maximum brightness is, that is, the more uniform the brightness distribution is.
However, since the amount of light that reaches regions far from the light source 51 is small, the amount of light reflected by the light admission planes 57a in the region is small. Accordingly, only a small amount of light exits from the first exit plane 53 in the regions far from the light source 51. This results in a reduced brightness and an uneven brightness. In other words, the brightness distribution is uneven. Japanese Laid-Open Patent Publication No. 2000-21224 discloses a wedge plate type light guiding plate having improved brightness and brightness uniformity. Japanese Laid-Open Patent Publication No. 2000-111900 discloses a flat plate type light guiding plate having improved brightness and brightness uniformity.
Specifically, in the light guiding plate 52 of the publication No. 2000-21224, the pitch of the grooves 57 is randomly varied. The randomly varied pitch and the wedge shape of the light guiding plate make the brightness uniform. On the other hand, in the flat plate type light guiding plate of the publication No. 2000-111900, the depth of the grooves increases as the distance from the light source 51 increases, while the pitch of the grooves is constant. Also, as the distance from the light source 51 increases, the inclination angle a gradually increases from 0.5xc2x0 to 3.5xc2x0 or the inclination angle xcex2 increases from 40xc2x0 to 60xc2x0.
In the light guiding plate disclosed in the publication No. 2000-21224 having a varying groove pitch, smaller pitch of the grooves 57 makes the brightness distribution uniform. However, if the pitch does not match the pixel pitch of the reflective liquid crystal panel 60, interference patters will appear. Also, since the groove pitch randomly varies, emission lines appear in regions where the pitch is greatly varied. Interference patterns and emission lines both degrade the visibility.
In the flat plate type light guiding plate disclosed in the publication No. 2000-111900, the apex angle xcex2 changes if the groove depth increases and the angle xcex1 or the angle xcex3 changes. In this case, since the angle xcex1 or the angle xcex3 changes, the brightness distribution cannot be easily made uniform. Since the apex angle xcex2 changes, the brightness does not increase even if the groove depth increases. If the apex angle xcex2 changes, manufacture of a mold for forming the light guiding plate will be complicated, and the accuracy of the mold will be degraded.
Accordingly, it is a first objective of the present invention to provide a wedge plate type light guiding plate for front light, which waveguide increases the brightness and improves the uniformity of brightness distribution while preventing interference patterns from appearing.
A second objective of the present invention is to provide a method for manufacturing a wedge type light guiding plate for front light, which method facilitates forming of an accurate mold and permits apex angles defined by peaks and bottoms forming grooves in the light guiding plate to be accurately formed as designed.
To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, the invention provides a wedge plate type light guiding plate for a front light mounted on a reflective display unit having a display portion. The light guiding plate guides light from a light source and irradiates the light onto the display portion. The light guiding plate has an incidence plane, an opposite end plane, an exit plane facing the display portion and a reflection-exit plane. The incidence plane permits light to enter the light guiding plate. The opposite end plane opposites to the incidence plane. Light entering the light guiding plate through the incidence plane exits through the exit plane toward the display portion. The reflection-exit plane opposites to the exit plane. Light reflected on the display portion passes through the exit plane and the reflection-exit plane. A plurality of grooves are formed on the reflection-exit plane. The grooves extend in a direction parallel to the direction in which the incidence plane extends, and are arranged at a predetermined pitch in a direction perpendicular to the direction in which the incidence plane extends. The depth of the grooves monotonously increases from the incidence plane toward the opposite end plane. Each groove is defined by intersecting first and second planes. The first plane is inclined such that the distance between the exit plane and the first plane increases as the distance from the light source increases. In each groove, the angle defined by the first plane and the exit plane is substantially equal to that of the other grooves. In each groove, the angle defined by the first plane and the second plane is substantially equal to that of the other grooves.
The invention also provides a method for manufacturing a wedge plate type light guiding plate for a front light mounted on a reflective display unit having a display portion. The light guiding plate includes an incidence plane for permitting light to enter the light guiding plate, an opposite end plane opposite to the incidence plane, an exit plane facing the display portion, and a reflection-exit plane opposite to the exit plane. A plurality of grooves having a saw-tooth cross-section are formed on the reflection-exit plane. The grooves extend in a direction parallel to the direction in which the incidence plane extends, and are arranged at a predetermined pitch in a direction perpendicular to the direction in which the incidence plane extends. The method comprising: preparing mold for molding the light guiding plate, wherein the mold has a plurality of peaks that correspond to the grooves, wherein each peak is defined by intersecting first and second planes, wherein the first plane is inclined such that the distance between the exit plane of the light guiding plate to be molded and the first plane increases as the distance from the incidence plane of the light guiding plate to be molded increases, wherein the first plane of each peak is parallel to the first planes of the other peaks, and wherein the angle defined by the first and second planes of each peak is equal to that of the other peaks; and injection molding the light guiding plate using the mold.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.